Inorganic glass, a transparent material, has heretofore been used for panel substrates and the like in displays such as liquid crystal display devices and organic EL display devices. However, inorganic glass has high specific gravity (weight), as well as lower flexibility and impact resistance. In this situation, use of a polyimide film which is lightweight and excellent in impact resistance, workability and flexibility for a panel substrate of a display apparatus has been taken into consideration.
For example, it has been proposed to use a polyimide obtained from an aliphatic tetracarboxylic dianhydride and a diamine for a display substrate (PTL 1). Also, it has been proposed to use a polyimide film synthesized from a diamine and an amide group-containing tetracarboxylic dianhydride derived from 1,2,4-cyclohexanetricarboxylic acid-1,2-anhydride for a display substrate (PTLS 2 to 4).
The panel substrate for a display apparatus requires high light transmittance. In a display apparatus, an image displayed by a device is observed through the panel substrate. Accordingly, the panel substrate also requires a small retardation in the traveling direction of light (thickness direction of the substrate).